Before selecting a frequency for your
AM station, be aware of potential interference at night on
certain channels.

Did you ever wonder why
some radio stations can
be heard for much
greater distances at
night?

This curious effect,
which often manifests as
nighttime interference,
is caused by a
phenomenon known as "skywave."
Skywave is essentially
radio waves from full
power AM stations that
travel upward, refracted
through the ionosphere
(from early evening
throughout the night
into morning) such that
radio signals sometimes
return to earth far from
where they originate.
It's especially
prevalent on
frequencies* that host
50,000-watt radio
stations, which pump up
to 200 times the energy
skyward, compared with
local and regional
stations.

Why just at night?
During the daytime, the
sun's radiation causes
atmospheric conditions
to change such that the
waves pass through the
ionosphere and into
space. At night when
that layer is cool, it
reflects them back. With
hundreds of radio
stations on a given
frequency, all bouncing
their skywaves
simultaneously, what is
heard is a garble of
background interference.

Some of the dynamics
that determine the level
of interference are
radio signal frequency,
ionosphere density and
the angle at which the
signal enters the
ionosphere. Because of
the earth's rotation in
relation to the sun, its
atmosphere constantly
changes. Solar storms
and sunspots from these
changes can also
contribute to skywave
interference.

How can skywave affect
your low-power AM radio
transmission?

The major effect is that
during the hours from
sunset to sunrise, the
softer fringe areas of
your low-power AM signal
can mix with the
interference to varying
degrees. The effect on
the signal can range
from complete
obliteration to the
sense that there is a
distracting sound in the
background. Closer in,
where your signal is
stronger, skywave
interference has
proportionally less
effect.

Skywave is heard on the
East Coast into the
morning daylight hours,
because out West, where
it is still dark, the
bounce continues. Out
West, it's the opposite
effect, with some
skywave taking hold well
before sunset, due to
eastern stations that
are already in the dark.